CYLINDER TESTING METHODS
Water Jacket Method:
The water jacket method for hydrostatic testing consists of loading a water filled cylinder into a sealed chamber (test jacket), which is also filled with water and is connected to a calibrated glass tube (burette) or Galiso's Electronic Expansion Measuring System. The Expansion Bowl (EEMS) was invented to replace the burette. The burette or Expansion Bowl is first zeroed, and the cylinder is then pressurized to its specified test pressure (test pressure requirements are contained in the U.S. Code of Federal Regulations, 49CFR173.34). This test pressure is held for a minimum of thirty seconds.
As pressure is applied to "inflate" the cylinder, the cylinder expands and forces water out of the test jacket and up into the Expansion Bowl or burette. After the thirty second test time has elapsed, the Expansion Bowl or burette is then read to determine the Total Expansion (in cubic centimeters) of the cylinder under test pressure. The test pressure is released and the cylinder "deflates". As the cylinder shrinks to it's approximate original size, water is allowed to drain back into the test jacket from the burette or Expansion Bowl. In most cases, the cylinder will not return to it's original size, having been slightly stretched by the pressurization process. This stretching is called the Permanent Expansion. The difference between the "Total Expansion" and the "Permanent Expansion" is called the Elastic Expansion. The Percent Expansion of the cylinder is determined by the following formula:
Percent Expansion = (Permanent Expansion / Total Expansion) X 100
When the Percent Expansion exceeds the predetermined limits for the cylinder being tested, the cylinder must be condemned and removed from service. A high percent expansion value is an indication that the cylinder metal has lost it's elasticity, or that there has been excessive thinning of the cylinder wall and that the cylinder is no longer safe for use.
All test records must be saved and maintained for the duration of the requalification
Plus (+) stamped cylinders may be filled to an additional 10 percent beyond the rating which is stamped on the cylinder shoulder. Star (*) stamping makes the cylinder eligible for an extended ten year re-test interval. The Water Jacket Method of testing compressed gas cylinders is the only hydrostatic test method that qualifies cylinders for filling to 10% over service pressure.
The procedures and requirements for plus stamping and star stamping are found in 49 CFR 173.302(c) for plus stamping, and 173.34(e)(16) for the star. REE values for common cylinders can be found in Compressed Gas Association Pamphlet C-5, "Cylinder Service Life, Seamless High Pressure Cylinders". This pamphlet is available from Galiso, or from the Compressed Gas Association at the address indicated above.
DIRECT EXPANSION METHOD (DE):
During the direct expansion test, the cylinder is completely filled with water and the test connection is then screwed into the cylinder neck. Water is pumped into the cylinder until the desired test pressure is achieved. (Test pressure requirements are contained in the U.S. Code of Federal Regulations, 49CFR173.34.)
The volume of water that must be pumped into the cylinder to reach the test pressure is measured to determine the Total Expansion. The volume of water that is expelled from the cylinder when pressure is released is measured to determine the Permanent Expansion.
Because air has a different compressibility factor than water, air trapped inside the cylinder will cause inaccurate test results. So, it is very important that the cylinder is completely filled with water to eliminate trapped pockets of air. The weight of the water contained in the cylinder, the test pressure, test volume(s) and temperature are used to determine the compressibility factor for calculation of the expansion values.
DOT Regulations prevent the Direct Expansion Method from being used to qualify cylinders for filling to 10% over service pressure and therefore forbid the Direct Expansion test method to be used to re-qualify plus ("+") stamped cylinders. The Direct Expansion test method is discussed in detail and example calculations given in CGA pamphlet C-1, "Methods for Hydrostatic Testing of Compressed Gas Cylinders".
While not generally practiced in the U.S., the U.S. Code of Federal Regulations permit that certain specified cylinders only (and used exclusively in non-corrosive service) do not require the total and permanent expansion to be calculated. For such cylinders, the Proof Pressure method may be used.
What is hydrostatic testing... courtesy of Galiso®